1. Field of the Invention
The present invention refers to devices and methods for controlling the application height of the material to be applied to a subgrade by means of a road finisher.
2. Description of Prior Art
A conventional road finisher in which the present invention can be used is shown in FIG. 3. The road finisher comprises a tractor 10 drawing a so-called "floating screed" 12. The floating screed 12 is attached to the tractor 10 by means of two drawarms 14, only one of said drawarms being shown in FIG. 3 due to the mode of representation. The front ends of the drawarms 14 are attached to the tractor 10 via vertically adjustable means 16. Said vertically adjustable means 16 can consist e.g. of hydraulic means comprising a piston and a cylinder as well as a valve device for controlling the position of the piston in the cylinder. The second ends of the drawarms 14 have rigidly secured thereto the floating screed 12.
When the road finisher is in operation, the drawarm 14 is attached to the tractor 10 only by means of the vertically adjustable coupling device 16. In order to permit transport of the road finisher without operation thereof, the second end of the drawarm can be arrested by means of an arresting device 18. When the road finisher is in operation, this arresting device 18 does not influence the movability of the second end of the drawarm 14.
The screed 12 is therefore connected to the tractor 10 via the drawarm 14 and a drawpoint, said drawpoint being adjustable via the vertically adjustable coupling device 16. According to the angle adjusted at the drawarm 14, the screed floats on the material to be applied.
The road finisher additionally includes a material spreading screw 20 by means of which the fed material to be applied is distributed over the width of the road to be built.
The application height of the material to be applied is normally supervised by a height control. Application height control during discharge of asphalt material by the road finisher is usually carried out with the aid of levelling systems using one of the following sensors for sensing the height: potentiometer sensor, ultrasonic sensors or laser receivers. The height sensor is fixedly connected to the screed through adequate holding means having integrated therein height adjustment means in most cases. In accordance with the prior art, the height sensor is normally arranged adjacent the side plate of the screed 12 approximately on one level with the material spreading screw 20. This mode of arrangement of a height sensor 30 is shown in FIG. 4. The height sensor 30 measures the height in relation to an associated reference 32. This reference is normally defined by a taut rope or by a taut wire. Alternatively, kerbs or the ground may serve as a reference. The height sensor 30 is arranged such that it can detect the height in relation to the reference arranged adjacent the road to be built.
FIG. 4 shows the drawarm 14 having the screed 12 attached thereto, for two different inclinations of the drawarm 14. As can be seen in FIG. 4, a change in the inclination of the drawarm 14 and, consequently, of the screed 12 can be caused by displacing the drawpoint by means of the vertically adjustable coupling device 16. Such an displacement of the drawpoint can be brought about by a valve adjustment of the vertically adjustable coupling device 16 or by surface irregularities. As can additionally be seen from FIG. 4, such a change in the inclination of the drawarm 14 and, consequently, of the screed 12 directly results in a corresponding change of height at the height sensor 30 in relation to the reference 32 associated therewith. As shown in FIG. 4, a change in the inclination of the drawarm 14 causes the detected height to change from h1, cf. left section of FIG. 4, to h2, cf. right section of FIG. 4. The measured value detected by the height sensor 30 changes from h1 to h2 immediately after the displacement of the drawpoint.
In FIG. 5, the basic structural design of a known height control is shown. The height sensor 30 detects an actual height value. This actual height value is compared with a target height value. A height controller 40 produces as a result of this comparison a control variable for the vertically adjustable coupling device, i.e. a control variable for the drawpoint valve 42 for adjusting the drawpoint. In the known method, the drawpoint is controlled by means of the detected height.
The correction of a control deviation is carried out in the known method as follows. The height controller 40 serves as a drawpoint controller and causes a displacement of the drawpoint. Due to the resultant adjustment of the inclination of the drawarm 14, the screed 12 floats up and down with a time delay. Still existing deviations from the target height value are corrected by the height controller by a renewed displacement of the drawarm. As soon as the drawarm is displaced, the height controller 40 again acts as a drawpoint controller until a stable adjustment of the drawpoint position has been obtained. The above-mentioned steps are repeated until the target height value is obtained.
In the control system described, the height sensor 30 is arranged adjacent the side plate of the screed approximately on one level with the material spreading screw, since a drawpoint displacement is not recognized directly when the height is sensed at the rear edge of the screed. The drawpoint control circuit would then not have a direct feedback and cause a movement up to the limit stop of the drawpoint displacement. This would result in a formation of waves in the material applied. It follows that, when height control according to FIG. 5, which represents the current prior art, is carried out, the height must not be sensed at the rear edge of the screed, although this would be the appropriate location for an exact detection of height.
In FIG. 6 a problem is shown which arises in the case of all finishers equipped with a levelling system in which the sensor for sensing the height is arranged at the position described hereinbefore. In FIG. 6, the screed 12 is shown schematically for various drawpoint adjustments. The drawarm is only shown schematically in the form of lines. Although the rear edge of the screed occupies three different height positions H1, H2 and H3, the height sensor 30, which is arranged at the position of the material spreading screw 20, measures in each case the same height H in relation to its reference 32. Due to temperature variations in the material to be applied or due to speed variations of the road finisher, slight tilting of the drawarm and, consequently, also of the screed around the measurement point of the height sensor 30 may occur. This tilting finds expression in a long-time drift of the height of the material applied. Experiments carried out in practice have shown that the height of the material applied may vary up to 1 cm over a period of several hours. In practice, this long-time drift must be corrected in that, e.g. the operating staff, regulary checks the height of the material applied.